Technical Field
Microtomes are used to cut thin sections, usually of organic tissue, for microscopic examination. This is a common process in histology labs and pathology. There are several types of microtomes. Commonly used are sledge microtomes and rotary microtomes. Usually the cutting stroke of a microtome is of fixed length.
A generic rotary microtome is being described in DE 3806269 C1 for example. The invention relates to a cutting stroke adjustment of such a rotary microtome for improved working conditions in histology labs.
Background Art
Rotary microtomes are characterized by a vertical cutting movement of the specimen holding device with the thereto attached specimen to be thin cut and a horizontal feed movement between cutting knife and specimen surface of the amount of the preset sectioning thickness. Depending on the type of the respective rotary microtome the horizontal feed can be arranged either by moving the knife or the specimen.
The vertical cutting movement of the specimen holding device, as part of a vertically moved carriage of a rotary microtome, is performed by a circular driving movement of a driving shaft, having at one end a crank arm with a thereto connected crank pin. That crank pin being freely rotatable engaged in a link piece guided in a motion link path thus resulting in a linear vertical movement of said vertical carriage.
Alternatively a corresponding crank pin can act together with other forms of horizontal guidances as described for example in DE 000004339071A1.
The circular driving movement can either take place manually by a handwheel with a handle or also be performed motorized thru a pulley connected to the driving shaft. When having a constant angular velocity of the driving circular movement the velocity of the resulting vertical movement has a sinus component.
The stroke length of the linear vertical movement of the specimen holding device with the thereto attached specimen corresponds to the diameter of the circle being described from the crank pin center on its travel around the center of the driving shaft.
The size of the cutting stroke length and thus the size of the circle diameter is for logical reasons elected to be in the range of the length of the specimen to be cut or respectively with some reserve distance in order to enable a problem free cutting into the specimen.
However, with a variation of tasks regarding the size of specimens it turns out that it is of disadvantage to have a cutting stroke length which is a fixed length by design. Therefore it was disclosed in the earlier publication DE000002253628 A to have a cutting stroke length adjustment to accommodate the different requirements from specimen sizes in routine histology on one side and in ultramicrotomy on the other side in order to cover both tasks with one type of apparatus.
This however did never gain acceptance.
But in the last years an increased demand occurred in the main field of usage of rotary microtomes, namely in routine histology, to cut besides the very often used specimen sizes of typically up to 30 mm length which are incorporated in so called standard cassettes also other specimen which are incorporated in so called macro-cassettes within the same laboratory and with the same microtomes. These macro-cassettes incorporate specimen sizes of around 70 mm length. Therefore in the first instance a trend started to increase the fixed cutting stroke length of most of the commercially available rotary microtomes from so far around 60-65 mm to stroke length of 70-75 mm without offering in the first place the already known possibility of a variable cutting stroke length of the apparatus.
This has rather negative consequences for the sectioning quality and ergonomy as well as for the efficiency of the working process, because on a worldwide basis more than 90% of routinely arising specimen have specimen sizes less than 30 mm. Microtomes with fixed stroke length of 60-65 mm were already of great disadvantage for the majority of specimen sizes, but this situation became worse with the introduction of the macro-cassette and the adaptation of the fixed stroke length to the maximum size.
The sectioning quality, namely to avoid compression of the sections and to avoid so called chatter on the sections produced, is greatly dependant from the cutting speed. Therefore it is a goal to maintain the lowest possible cutting speed with having a high specimen throughput and a high sectioning sequence. This means for rotary microtomes absolutely to provide a short cutting stroke length for the majority of specimen. In addition it is in view of ergonomy and work place efficiency important to avoid unnecessary phases of idle movement with the type of specimens used most. To offer nevertheless universal apparatus the demand for a variable cutting stroke length raises again.
In DE 102008016165 A1 this requirement is regarded to a certain extent.
However, the microtome described there has only motorized function and there are no means for a direct mechanical coupling of a handwheel with handle and the resulting sectioning process.
This is of disadvantage, because it is known that the user does not want to give up the manual operation in the phase of first cuts into the specimen and also for delicate specimen. Therefore a backlash-free mechanical coupling is of importance for an adequate sensoric feedback to the manual operating user and of unvaluable worth in case of difficult specimen material with enhanced sectioning requirements.
Hence rotary microtomes with mechanically coupled handwheels are preferred even when they are operated motorized optionally.
In DE 102008031137 A1 a cutting stroke adjustment for a rotary microtome is described. There is also mentioned the difficulty with rotary microtomes which arises from necessary compensating weights in case of a variable cutting stroke adjustment.
Commonly, rotary microtomes have a compensation mass for compensation of vertically movable mass which generates together with the crank arm of the crank pin a moment of force. The compensation mass being connected to the drive shaft generates with its distance of the center of mass to the drive axle a countermoment of force of same amount but with 180° phase shift. From that results the desired and necessary balance of the driving movement. With a variable adjustable cutting stroke it is therefore in any case necessary to have a corresponding alignment of the effect of the compensation mass. This can be achieved either by changing the mass itself or by changing the effective lever arm of force.
In DE 102008031137 A1 is a cutting stroke adjustment described as well as an accordingly necessary compensation mass adjustment. Both adjustments being conducted from outside with tools and in sequence. For adjusting the cutting stroke and the compensation mass a locking mechanism is in place in order to have access to the positions of the adjusting means thru openings of the microtome housing.
The means described in DE 102008031137 A1 meet admittedly the basic requirements for an adjustment of the cutting stroke matching the specimen sizes in use, however they are cumbersome and time-consuming in their execution.
This is of gross disadvantage in the routine operation in a histology laboratory, particularly if several times a day there is a need to change from regularly used small specimens to large specimens and back.